Repro:
1) Create a Device application (DevApp1)
2) Add another device application (DevApp2) in the same solution.
3) Add Project reference (DevApp2) in DevApp1.
4) Build the solution.
5) Open configuration manager and uncheck the build option for DevApp2.
6) Open Program.cs( of DevApp2)
7) Build the solution, build fails.
8) You will get the error message “The OutputPath property is not set for this project. Please check to make sure that you have specified a valid Configuration/Platform combination. Configuration='' Platform='AnyCPU' “
Workaround:
1) Open the configuration Manager.
2) Check the build option for the DevApp2( All the P2P references)
3) Build the solution. Build will succeed.
4) Open the configuration manager.
5) Uncheck the build option for DevApp2.
6) Don’t click on any of the DevApp2 hierarchy otherwise the user will end up into the same error message.
转自msdn